1. Field of the Invention
The present invention relates generally to an exhaust gas recirculation (EGR) control system for reducing the production of nitrogen oxides (NO.sub.x) by combustion of an air-fuel mixture in an internal combustion engine by recirculating a controlled amount of exhaust gases of the engine into the engine intake air and particularly to an EGR control system which is provided with an additional expedient for controlling the amount of recirculated exhaust gases in cooperation with an EGR control valve.
2. Description of the Prior Art
As is well known in the art, in a conventional EGR control system the amount of recirculated exhaust gases has been controlled in accordance with the amount of engine intake air by operating the EGR control valve by a vacuum signal such as, for example, a venturi vacuum produced in a carburetor. Although the degree of opening of the EGR control valve is correctly controlled by the venturi vacuum representing a function of the flow rate of engine intake air, since the temperature and pressure of exhaust gases passing through the EGR control valve variously vary in accordance with an operating condition of the engine, it has been difficult to control the EGR rate, that is, the ratio of the flow rate of recirculated exhaust gases to the flow rate of engine intake air, to a predetermined value at all times.
It is necessary to reduce the error of the accuracy of exhaust gas recirculation control as much as possible for effectively reducing the production of nitrogen oxides concurrently with insuring the stability of operation of the engine. This is especially necessary when the exhaust gas recirculation is performed at an EGR rate of a high value.
From the fact that the amount of exhaust gases emitted from each of the combustion chambers of an engine having a plurality of combustion chambers is exactly at a constant rate to the total amount of exhaust gases emitted from all the combustion chambers of the engine, the inventor has discovered that it is possible to make the error of the control of the EGR rate smaller by mainly recirculating exhaust gases from a part of the combustion chambers into the intake passageway of the engine and by controlling the EGR amount on the basis of the amount of the exhaust gases from the part of the combustion chambers.
To take as an example an engine having four combustion chambers, the amount of exhaust gases emitted from one combustion chamber of the engine is exactly 25% of the amount of exhaust gases emitted from all the combustion chambers. Accordingly, if all exhaust gases from the one combustion chamber are recirculated into the intake passageway of the engine, the EGR rate exactly becomes 25%. If only a part of exhaust gases from the one combustion chamber is recirculated, the EGR rate becomes below 25%. Conversely, if a part of exhaust gases emitted from the remaining combustion chambers is recirculated together with all exhaust gases from the one combustion chamber, the EGR rate becomes above 25%. Since the average EGR rate becomes frequently about 25% in current EGR systems which recirculate a great deal of engine exhaust gases, it is very significant in accuracy of the control of the EGR rate to take 25% as a basis of the EGR rate.